The present invention relates to arrangements for compensating inaccuracies occurring in measuring devices which include an optical path having an optical transmission characteristic which varies as a function of an electrical parameter being monitored.
For example, magneto-optic current transformers (MOCTs) vary the optical attenuation occurring in an optical path as a function of an alternating current being monitored. Typically, such a transformer is connected in an optical path having an electrically driven light source at its input end and a light receiver at its output end, with a circuit being connected between the light receiver and the light source for suppressing low frequency or long term, changes in the optical energy at the output end of the optical path.
FIG. 1 shows a known circuit for a magnetooptic current transformer (MOCT) 2. MOCT 2 is provided with a light entry port, a light exit port, a light transmission path between the ports, and means responsive to an electric current for influencing the optical characteristic of the light transmission path such that the relation between optical power, or light intensity, supplied at the light entry port and the optical power, or light intensity, appearing at the output port is a function of the instantaneous current amplitude
In the circuit of FIG. 1, an input fiber 4 is coupled to the entry port and an outlet fiber 6 is coupled to the exit port of MOCT 2.
Essentially, the circuit provides an output voltage, V.sub.out, corresponding to the alternating current acting on MOCT 2.
The circuit includes a photoemitter 8, which may be an infrared-emitting LED, for supplying optical energy to fiber 4 and a photoreceiver 10, such as a PIN diode photodetector, connected to receive optical energy emerging from fiber 6.
Photoreceiver 10 is connected to the input of a current-to-voltage converter stage 12 and to a source of a constant reference current, I.sub.o. Stage 12 is composed essentially of a differential amplifier having a resistive feedback connection and operates to produce output voltage V.sub.out proportional to the optical power received by photoreceiver 10.
The remainder of the illustrated circuit serves to suppress long term variations in the optical power received by photoreceiver 10 by adjusting the optical power emitted by photoemitter 8 in a manner to oppose long term variations in the optical power received by photoreceiver 10.
For this purpose, the circuit further includes an integrator 14 connected to receive V.sub.out and a voltage-to-current converter 16 which includes a transistor Q1 and has an exponential transfer characteristic. These two units function in a known manner to control the optical power introduced into fiber 4 in a manner to offset long term, or low frequency, changes in the optical power received by photoreceiver 10.
The output signal from integrator 14 is supplied by a voltage divider composed of resistors R.sub.3 and R.sub.4, the center tap of which is connected to the base of a transistor Q1 to provide the input voltage to converter 16.
Analysis of the circuit shown in FIG. 1 indicates that with respect to the relation between the current being monitored and V.sub.out, and without the exponential voltage-to-current converter 16, the circuit behaves like a single pole high pass filter having a corner frequency, cut-off frequency which varies as a direct function of the total optical attenuation, or reduction in optical power, between the output of photoemitter 8 and the input of photoreceiver 10. This corner frequency variation creates a corresponding phase shift in V.sub.out at the frequency of the current being measured. When V.sub.out is used for the generation of average power reading, this phase shift will result in a possibly significant error.
The use of a voltage-to-current converter 16 having an exponential characteristic reduces the rate of corner frequency variation with attenuation but, in practical devices, an attenuation variation of 10:1 still produces a corner frequency variation of 2:1.